A subdomain is a website url, that's under a main, or a root domain. One example is something.domain.com, in which domain.com is the main domain address, while "something" is the subdomain below it. Each subdomain can have its own site and records i.e. aside from using the domain name as a part of the whole web address, a subdomain can be totally independent and even hosted by a separate provider. Employing a subdomain could be very handy if you'd like to have different sites that are related in some way. As an example, you may have a web-based store under the main domain plus a forum under a subdomain in order to offer means for your clients to ask questions and share opinions regarding your products. The benefit of using a subdomain instead of a subfolder is that the web sites will be separated, so you're able to update one of them or perform maintenance without affecting the other.